fix(dotenv): encode []string as CSV instead of Go bracket notation

[lcensies]

Bug description

fmt's %v on []string produces "[a b]" which gotenv reads back as a
literal string, breaking round-trip.

Example:

cfg.Set("Hosts", []string{"172.12.5.2:3434"})

Gets serialized to

HOSTS=[172.12.5.2:3434]

Which fails during backward deserialization

Fix

Serialize string list as csv-like format

+               case []string:
+                       valStr = strings.Join(v, ",")

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}